The skin is always exposed to various irritations from the external environment. The stratum corneum of the skin has barrier functions of preventing invasion of these irritations or foreign matters from the outside and preventing transpiration of water within the body. It has been known that water content in the stratum corneum of the skin of persons or animals in which such barrier functions are reduced is decreased. For example, it has been reported that the water content in the stratum corneum of the skin is decreased in patients with atopic dermatitis (refer to Non-patent Document 1) or with aging (refer to Non-patent Document 2).
As a method for maintaining or improving moistening property of the skin, a method in which the barrier functions of the stratum corneum are supplemented with a blocking agent such as a vaseline ointment or a water-in-oil emulsion, a method in which the water content of the stratum corneum is supplemented with a moistening agent such as sorbitol or glycerin, a method in which skin inflammation is suppressed with an antiinflammatory agent such as glycyrrhizic acid, a method in which skin cells are activated with, for example, vitamins or hormones, and the like have been so far used (refer to Non-patent Document 3).
Citrulline is known to be a constituent of NMF (natural moisturizing factor), which is an epidermal moisturizing component (see non-patent reference 4). It has been reported that a topical administration of Citrulline in combination with arginine, ornithine etc. can prevent or treat nerve sensory symptoms such as atopic skin and the like (patent reference 1). However, it is not known that oral ingestion of citrulline or a salt thereof can prevent or improve a dry state of the skin.    Patent Document 1: Japanese Translation No.505822/1997 of PCT Application    Non-Patent Document 1: “Archives of Dermatology”, 1991, vol. 127, p. 1689    Non-Patent Document 2: “Journal of Investigative Dermatology”, 1990, vol. 95, p. 296    Non-Patent Document 3: “Fragrance Journal”, 1999, vol. 10, p. 29    Non-Patent Document 4: “Journal of the Society of Cosmetic Chemists”, 1984, vol. 35, p. 183